The present invention relates to optical apparatus, and particularly to apparatus using liquid crystals for shaping the spatial intensity profile of optical beams having designated wavelengths.
The invention is especially suitable for use in providing a soft aperture for shaping the intensity of laser beams utilizing the properties of cholesteric liquid crystals. The term "liquid crystals" as used herein includes crystals both in the solid and fluid state. The term "cholesteric" is used generically to mean liquid crystals which have chirality, whether pure cholesteric compounds or nematic materials mixed with chiral additives.
Birefringent elements have been proposed for use as soft aperture devices. They are made by the use of materials having retardance which decreases radially from the center to the edge of the device, usually by varying the thickness of the material. Birefringent crystals both solid and liquid have been used in such devices. See S. D. Jacobs, "Liquid Crystals As Large Aperture Waveplates and Circular Polarizers", SPIE 307,98-105, 28 Aug. 1981; D. Giuliani, et al., "Radial Birefringent Element And Its Application To Laser Resonator Design", Opt. Lett. 5, 491 (1980); and J. M. Eggelston, et al., "Radial Intensity Filters Using Radial Birefringent Elements", J.O.S.A. 71, 1264 (1981). Soft apertures have also been made using variably absorbent glass. See Hudson, U.S. Pat. No. 3,465,347 issued Sept. 2, 1969.
Liquid crystal devices providing notch or blocking filters have been designed to selectively reflect laser light having a limited wavelength spectrum, but do not vary the spatial intensity of the beam to provide an apodizing effect or act as a soft aperture. See U.S. Pat. No. 3,679,290 issued July 25, 1972 and U.S. Pat. No. 3,711,181 issued Jan. 16, 1973 and the following articles, J. Adams et al., "Cholesteric Films As Optical Filters", J. App. Phys. 42(10) 4096 (1971); F. J. Kahn, "Cholesteric Liquid Crystals For Optical Applications", Appl. Phys. Lett., 18(6), 231 (1971); S. D. Jacobs et al., "Liquid Crystal Laser Blocking Filters", LLE Review 15, 30 (1983); and S. D. Jacobs, et al., "Liquid Crystal Laser Blocking Filters", presented at the Annual Meeting of the Optical Society of America, New Orleans, La., Oct. 18, 1983.
Through the use of cholesteric liquid crystals bodies in accordance with the invention, the spatial profile of designated wavelengths of optical radiation can be tailored across almost any aperture area and differently in different areas of the aperture. Oftentimes the beams to be shaped or apodized propagate along the same or closely adjacent paths, for example the beams may be colinear beams. In addition to being of different wavelengths, the beams may have different cross sections which are overlapping. The optical apparatus provided by the invention makes it possible to shape or apodize such beams of different wavelengths and cross sections without interference, i.e. the apparatus acts as a soft aperture for one of the beams without apodizing the other. The shapes of the beams can be tailored to different dimensions, again without interference and without requiring spatial beam separation. Other features and advantages of the optical apparatus for spatial intensity shaping which is provided by the invention are: Capability of shaping beams having any arbitrary input polarization; Compact size and ease of fabrication so as to shape beams regardless of their cross sectional dimension; The ability to shape beams over a wide range of desigated wavelengths, for example from 300 nanometers (nm) to 5000 nm; Capability of operating at high beam power, since shaping is accomplished through selective reflection and not absorption; High transmittance and low insertion loss for the portions of the beam which are not modified in intensity (the unapodized portions of the beam); and Minimal wavefront distortion to the transmitted beam, since the liquid crystal and the substrates which comprise the apparatus can be index matched.